This invention relates to magnetic resonance (MR) apparatus. More specifically, this invention relates to a means for sensing whether the upper assembly of a two-piece radio frequency (RF) head coil in MR scanner is properly positioned for performing MR scans.
The magnetic resonance phenomenon has been utilized in the past in high resolution MR spectroscopy instruments by structural chemists to analyze the structure of chemical compositions. More recently, MR has been developed as a medical diagnostic modality having application in imaging the anatomy, as well as in performing in vivo, non-invasive, spectroscopic analysis. As is now well known, the MR resonance phenomenon can be excited within a sample object, such as a human patient, positioned in a homogeneous polarizing magnetic field, by irradiating the object with radio frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having a cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or a different RF coil is used to detect the MR signals emanating from the patient volume lying within the field of the RF coil. The MR signal is usually observed in the presence of linear magnetic field gradients used to encode spatial information into the signal. In the course of a complete MR scan, a plurality of MR signals are typically observed. The signals are used to derive MR imaging or spectroscopic information about the object studied.
A whole-body MR scanner used as a medical diagnostic device includes a magnet, frequently of solenoidal design, to produce the polarizing magnetic field. The bore of the magnet is made sufficiently large to accommodate RF, gradient, and shim coil assemblies, as well as the torso of a patient to be examined. The scanner also includes a table which supports a cradle used to retrievably position the patient within the bore of the magnet. The table is aligned longitudinally with the bore of the magnet and disposed at the same height to facilitate the advancement of the cradle between a home position when the cradle is on the table and an advanced position when the cradle is in the magnet. A bridge structure in the bore supports the cradle and the patient when the cradle is in the advanced position.
Typically, a whole-body MR scanner is provided with two RF coils of different sizes. The larger coil, termed the body coil, is constructed on a cylindrical coil form having a diameter sufficiently large (e.g., 60 cm) to accept the torso of a typical patient. The smaller coil also having a cylindrical geometry, but smaller diameter, is used for imaging the head. A particularly advantageous head coil is constructed from two separable sections, as disclosed and claimed in commonly assigned U.S. patent application Ser. No. 551,626, which is incorporated herein by reference.
Briefly, the head coil is made up of an upper and a lower assembly which together comprise the cylindrically configured head coil. Each assembly supports one half of the RF coil circuit. The lower assembly is secured to a patient cradle, while the upper assembly is slidably mounted to a cradle dolly, which resides on the bridge within the bore of the magnet. This configuration enhances patient comfort, since the patient's face remains unobstructed until just before the MR study begins, when the upper half is pulled into place on top of the lower assembly to form the complete coil. When the coil assemblies are in place, the coil circuit halves are inductively coupled and are capable of functioning as a single resonant circuit at substantially the same frequency when one of the circuits is energized.
It is desirable, when imaging the head, to detect whether the upper coil assembly is in place prior to the start of the MR study. It is further desirable to detect whether the upper coil assembly remains in place during the study. For example, the assembly could be inadvertently pushed out of position by the patient. Advantageously, detection should be accomplished through an existing RF coaxial cable coupled to the RF coil circuit in the upper assembly, thereby to minimize the number of connections. The sensing function should not interfere with operation of the RF cable as a means for applying RF energy to the coil and transmitting MR signals from the study object to an MR receiver.
It is, therefore, a principal object of the invention to provide means for sensing the position of the upper assembly of an RF coil through existing RF cable and without interference with normal operation of the cable.